This invention relates to improved apparatus for testing the strength of a specimen of a selected material, such as for example a particular rock, concrete, or the like, under compressive forces. Certain features of the invention have been shown in Disclosure Document No. 036455 filed Oct. 31, 1974 in the U.S. Patent Office.
Conventional machines for testing material strength under compression take the form of presses having a pair of spaced platens between which the specimen to be tested is placed, and which are adapted to be pressed toward one another and against the specimen with great force, by hydraulic piston and cylinder mechanisms or other actuating means. The stresses developed in the specimen during such compression are monitored by a strain gauge or other equipment, and the point of breakdown of the material is recorded, to thus obtain an indication of the capacity of the material to withstand forces of different intensities.
In a somewhat different type of test of material strength, the specimen may be confined between two such platens while force is applied to the specimen in a lateral direction, that is, transversely of a line extending between the platens. Such lateral force may be applied in conjunction with compressive movement of the first mentioned platens, or without such movement and with the specimen merely being confined between the platens. The lateral force may be applied by a hydraulic fluid, or by additional platens acting transversely against the specimen.
A major difficulty which has been encountered in prior material testing equipment of this general type has involved inaccuracies in test results caused by frictional forces which are developed between the platens and the surfaces of the test specimens which are engaged by the platens. More specifically, under the high compressive forces which are encountered between the platens and specimen, the friction between the engaged faces of the platen and specimen causes the platen to resist very substantially lateral or transverse expansion of the specimen at those engaged faces as the specimen is compressed. Similarly, if the overall forces applied to the specimen are primarily lateral, the frictional interengagement of the platens and specimen may resist lateral contraction of the specimen which would otherwise occur at the interengaging faces of the specimen and platens. The platens are normally formed of a material which will not deform as much as the specimen under test pressures, and which by friction tends to restrain the engaged surfaces of the specimen to only such very limited change in dimension as is experienced by the platen itself. This restraint offered by the platens to deformation of the specimen under load necessarily alters the stress distribution pattern within the specimen, and adversely affects the accuracy of the test results, usually by giving an indication of greater strength in the specimen than it actually has.
In an attempt to eliminate such resistance by the platen to deformation of the sample being tested, it has heretofore been proposed that a relatively soft material be interposed between the platen and specimen for transmission of the compressive forces therebetween. For example, rubber has been utilized between the platen and specimen, as have Teflon, paste, building boards, and other similar materials or combinations of materials. However, these substances have by virtue of their relatively soft characteristics tended to squeeze out of the spaces between the platens and the test specimen, and in so squeezing out have exerted expansive forces on the engaged surfaces of the specimen which tend to increase its lateral deformation and, again, substantially alter the response of the specimen to the applied load. The test results achieved in this way may be as inaccurate as those achieved when the platen directly engages the specimen, though the effect on the results is usually reversed by tending to cause development of unintended tensile stresses in the specimen, and earlier breakage under compression than should occur.